If you really care about your privacy in internet communication, you probably will arrive at Kontalk Messenger. Kontalk is a XMPP-based secure communication platform, it supports end-to-end encryption and federation, and is free software for both the client and the server programs. If you interested enough, read this FSFE article about privacy and Kontalk FAQ. Kontalk is currently available in Android (phone), GNU/Linux, MacOS, and Windows (desktop). This article explains how to install Kontalk Desktop in Ubuntu & Linux Mint.

Requirement

Kontalk Desktop is basically a Java program. So to run it, the user needs Java Runtime Environment. Kontalk desktop requires JRE 8. To install this requirement in Ubuntu and Linux Mint, run this command line in Terminal:

2) Extract Package

Extract the zip package into a folder in your $HOME in desktop. This will create a folder named KontalkDesktop-3.1.2. This creates two items inside that folder: a lib folder and a KontalkDesktopApp.jar program.

3) Run Kontalk Program

4) Export Kontalk Keys (from phone)

In your phone, open Kontalk program > Settings > Maintenance > Export personal key > choose a folder of your phone > press OK. This will create a new file named kontalk-keys.zip on your phone memory.

5) Import Kontalk Keys (into desktop)

Send the kontalk-keys.zip from your phone to your desktop GNU/Linux. You may send it either with MTP (cable) or with another way. Place the file to the KontalkDesktop-3.1.2 folder, the same folder with the .jar program.

6) Configure Connection

If you encounter some error saying Kontalk can not connect to the server, open Preferences > Account try to check Disable certificate validation > click Save & Connect. This is a workaround I found myself, and if you know better workaround please let me know.

7) Connect

Now, you should be connected successfully to the Kontalk server. The sign of being connected is the "Connected" message on the bottom-left corner of the window.

8) Go Chatting

You can chat with your friends with Kontalk Desktop now.

Additional

You can add new contact by clicking on blue-circle button on the bottom.